The network communications industry has selected the use of twinax wire-pair cable assemblies for the transport of high data rate information. One of the challenges of a twinax wire-pair is that the shield tape that surrounds the twinax wire-pair can interact with the wire-pair in a destructive fashion, producing large resonant spikes (in its transfer function response) that can restrict its intended bandwidth and not support the intended application.
For applications requiring a high data rate with low latency performance such as in Storage Area Networks (e.g., Fiber Channel over Ethernet) and High Performance Computing (e.g., Infiniband), the copper media selected for connectivity must have a very high bandwidth capacity, such as with twinax cable, to support the un-modulated baseband signal. To obtain low latency while having low power dissipation, baseband digital communication is typically used instead of a complex modulation scheme requiring sophisticated coding techniques. A drawback is media analog bandwidth. For example, in order to support 10 Gbps (Gigabit/sec) data communication; the media must support at least 5 GHz (typically around 7 GHz) of analog bandwidth. The twinax cable industry has demonstrated that twinax cable can support this data rate and even beyond these data rates (see references to single data rate Infiniband and double data rate Infiniband). In order to achieve these bandwidths, the cable design is refined to address performance parameters in this high frequency range.
For example, to minimize insertion loss, the copper wire surface must be exceptionally smooth (to reduce surface roughness loss), it should be plated with silver (to enhance the smoothness via plating and reduce corrosion; silver also has a conductivity higher than copper), and the wire dielectric must have very low loss (i.e., low dielectric loss or loss tangent). The shield tape plays a critical role in the electro-magnetic wave characteristic that the wire-pair forms. Additionally, there are a number of cable applications that utilize twinax wire-pairs. SFP+ cable utilizes two twinax wire-pairs in its construction (called a one-lane media (where a lane refers to a transmit pair and a receive pair) and also called CX1 standing for “copper times 1 lane”). 40 Gbps Ethernet cable utilizes 8 twinax wire-pairs (CX4) and 100 Gbps Ethernet cable utilizes 20 such twinax wire-pairs (CX10) in its cable construction.
FIGS. 1A-1B illustrate the twinax cable construction for a typical SFP+ CX1 application. FIG. 1A shows a perspective view of a SFP+ cable 100 and 1B shows a cross-sectional view of the cable 100. Each of the two twinax wire-pairs 102 is shielded with a shield tape 104 that is either spirally or longitudinally wrapped about it. A shield tape 104 generally includes a thin sheet of aluminum metal (approximately 1 mil thick) laminated upon an insulating substrate (e.g., polyethylene plastic). (Spiral wrapping is generally preferred due to its superior mechanical properties.) For electrical properties like return loss, it is important to keep the shield in close proximity to the wire-pair 102 as well as keeping the wire-pair 102 together, especially during bending and normal handling of the wire-pair 102. A disadvantage of the spiral wrapped shield is that a resonance can occur. A longitudinally wrapped shield over the twinax wire-pair is preferred from an electrical (differential mode propagation) standpoint because this resonance does not occur but a cable with a longitudinally wrapped shield has poorer mechanical properties, such as separation of pairs during bending resulting in a change of impedance and higher return loss. From a common mode propagation viewpoint, the longitudinally wrapped twinax cable has a lower attenuation which can be problematic due to mode conversion within the cable or within the connector. In the choice between longitudinal or spiral wrap of the shield tape one must consider multiple cabling parameters. The twinax cable assembly 100 (including two of these shielded twinax wire-pairs) is then further shielded with both a shield tape 106 and a wire braid 108. The shields have two different purposes, one is for a signal ground and the other is for a safety ground. The twinax wire-pair shield 104 is used for the signal ground and the outer shield 106, 108 is used for the safety ground. The twinax wire-pair shield tape 104 wraps about a drain wire 110 which should, but doesn't always, have good conductivity to the shield tape 104. The drain wire 110 is used to bridge the signal ground from the cable into the connector signal ground plane.
FIG. 2A illustrates a wire-pair 102 of a twinax cable whose shield 104 is spirally wrapped. FIG. 2B shows a cross-sectional view of the SFP+ CX1 cable 100. The main purpose of the drain wire 110 is to connect the shield ground of the cable to the signal ground of the connector. Since an appreciable amount of the signal is contained (coupled to and propagated within) on the shield 104, how the shield 104 terminates onto the connector is critical to the signal integrity, hence the importance of the high frequency low impedance connection between the drain wire 110 and the shield 104. FIG. 2C illustrates the drain wire 110 to shield 104 contact problems that can occur in a twinax cable 100. When a high frequency low impedance connection is not made (lower views in FIG. 2C), the return loss is increased which can lower the performance of the cable 100.
FIGS. 3A-3C describes the electro-magnetic (EM) field lines that form in a twinax cable wire-pair 102 as well as how the current within the shield 104 travels. In FIG. 3A, the shield 104 is spirally wrapped tightly around the wire-pair 102 and drain wire 110. The shield tape 104 is tightly wrapped around the wire-pair 102 and the drain wire 110 in order to consistently keep the wire-pair 102 together in order to maintain the characteristic impedance (and get good return loss performance) and provide a low resistive contact between the drain wire 110 and the shield 104. FIGS. 3B and 3C show the electric and magnetic field lines within the twinax cable wire-pair 102. The problem is that the shield current does not exclusively follow the EM wave that is traveling longitudinally down the cable. Portions of the energy follow the shield tape 104 in a spiral path and portions will follow the EM field longitudinally (through capacitive coupling within the overlap region). The portion of the signal that follows a spiral path creates a resonance at a frequency corresponding to the overlap periodicity. For example, if the distance between the overlaps is of length 7 mm (and is periodic) the resonant frequency has a fundamental frequency of 28.6 GHz and has second and third harmonics of 14.3 and 7.15 GHz. The resonance that occurs near 7.15 GHz dramatically increases the attenuation of a twinax cable in the vicinity of this resonant frequency. The frequency range where the attenuation is generally effected is from a frequency of 5 times lower frequency than the resonance (hence 1.4 GHz and higher).
FIG. 4 shows that for a longitudinally applied shield (FIG. 4A), the current in the shield travels in the longitudinal path along with the EM wave formed between the conductors. This results in an insertion loss transfer function that performs well as shown in FIG. 4C, although it also includes the aforementioned mechanical and common mode problems. The spirally wrapped shield shown in FIG. 4B can perform poorly as shown in FIG. 4D, with a resonance peak frequency that is proportional to the shield tape overlap to overlap distances.
What is needed in the art is a twinax cable including spirally wrapped shields around respective wire-pairs with an improved attenuation spectrum, and possibly other improved electrical and mechanical characteristics which may increase the useable length of the cable in a number of applications.